1. Field of the Invention
This invention relates to a method of making a photosensitive element which, upon imagewise radiation exposure and wet development, can be made into a lithographic printing plate by the silver complex diffusion transfer process. In particular, this invention relates to a process for coating the layers in the photosensitive element which improves the sensitometric performance of the finished plate.
2. Background of the Art
Photosensitive elements which can produce lithographic printing plates are well known in the art as represented by U.S. Pat. Nos. 3,870,479; 4,621,041; 4,160,670; 4,784,933; 4,510,228; 5,202,218; and 5,281,506. These patents disclose printing plates which are made by the imagewise exposure of a photosensitive element and the formation of a silver complex layer on the surface of the medium to differentially hold or repel inks.
These photosensitive elements essentially comprise a substrate, a negative-acting silver halide emulsion layer, and a surface receptor layer which has deposition nuclei (e.g., palladium particles). Upon exposure of the silver halide emulsion layer, the exposed photosensitive element is developed with black and white developing agents in the presence of a silver halide solvent (e.g., thiosulfate, thiocyanate, aminoalkanols, etc.). The development may occur using either an alkaline developing solution or activator solution. If an activator solution is used, developing agents are incorporated directly into the photosensitive element. In the exposed portions of the photosensitive element the silver halide is reduced to a black silver image. In the unexposed portions, the silver halide solvent causes the silver halide to diffuse. At least some of the silver halide diffuses to the receptor layer where the deposition nuclei and the developer together reduce the silver halide to a metallic silver. Complexing the metallic silver with an organic complexing agent renders the surface of the element hydrophobic (oleophilic).
These photosensitive elements have, to the knowledge of the present inventors, been made by coating the silver halide emulsion layer and subsequently applying the deposition nuclei layer at an elevated temperature onto the first coated silver halide emulsion layer. In the manufacturing experience of the inventors on diffusion transfer printing plates, the deposition nuclei layer has been traditionally coated at temperatures of about 35.degree.-55.degree. C. The deposition nuclei coating solution may be either an organic solvent based system or aqueous based system having little or no organic solvents.
In a review of the prior art, all examined references were silent as to the coating temperature of the receptor layer with deposition nuclei.